Re: [PATCH] ftrace: Fix off-by-one fentry site disable in ftrace_free_mem()

On Sun,  2 Aug 2026 20:08:35 -0700
Josh Poimboeuf <[email protected]> wrote:

> When a module's init text is freed, do_init_module() calls
> ftrace_free_mem() with a half-open [start, end) range.  However the
> ftrace_cmp_recs() comparator treats the upper bound as inclusive, as all
> its other users do, passing 'ip + size - 1'.  So ftrace_free_mem() can
> delete a record sitting exactly at 'end', which is outside the freed
> range.
> 
> For a kernel without CFI or IBT, the first record of a function is at
> the function start, which for the first function in a module is also the
> base of its text allocation.  As the module allocator packs its regions,
> that address is often the 'end' passed by a neighboring module's
> do_init_module(), causing the first function's ftrace location to get
> disabled, preventing an attempt to livepatch it:
> 
>   livepatch: failed to find location for function 'pcspkr_probe'
> 
> Convert the exclusive end to the inclusive 'end - 1' the comparator
> expects, and return early for an empty range to avoid the subtraction
> from underflowing when the init text size is zero.

Nice catch.


> 
> Fixes: 42c269c88dc1 ("ftrace: Allow for function tracing to record init functions on boot up")
> Signed-off-by: Josh Poimboeuf <[email protected]>
> ---
>  kernel/trace/ftrace.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
> index f93e34dd2328..7d8b736f0d86 100644
> --- a/kernel/trace/ftrace.c
> +++ b/kernel/trace/ftrace.c
> @@ -8293,8 +8293,11 @@ void ftrace_free_mem(struct module *mod, void *start_ptr, void *end_ptr)
>  	struct ftrace_init_func *func, *func_next;
>  	LIST_HEAD(clear_hash);
>  
> +	if (start >= end)
> +		return;
> +
>  	key.ip = start;
> -	key.flags = end;	/* overload flags, as it is unsigned long */
> +	key.flags = end - 1;	/* overload flags, as it is unsigned long */

I'd like to keep this consistent with lookup_rec().

>  
>  	mutex_lock(&ftrace_lock);
>  

Can you do this instead?

-- Steve

diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index 6c47a94f5924..dbb0fc2928d8 100644
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -8296,7 +8296,8 @@ static void add_to_clear_hash_list(struct list_head *clear_list,
 void ftrace_free_mem(struct module *mod, void *start_ptr, void *end_ptr)
 {
 	unsigned long start = (unsigned long)(start_ptr);
-	unsigned long end = (unsigned long)(end_ptr);
+	/* end is inclusive and end_ptr is exclusive */
+	unsigned long end = (unsigned long)(end_ptr) - 1;
 	struct ftrace_page **last_pg = &ftrace_pages_start;
 	struct ftrace_page *tmp_page = NULL;
 	struct ftrace_page *pg;